Various proposals have been made so far on polymerization catalysts for conjugated dienes, and the polymerization catalysts play a very important role in industrial fields. In particular, various polymerization catalysts which provide a high cis-1,4-linkage content have been studied and developed to obtain conjugated diene polymers with enhanced performance in thermal and mechanical characteristics. For example, complex catalyst systems containing as a main component a compound of a transition metal such as nickel, cobalt, and titanium are known, and some of them have already been widely used in industrial applications as polymerization catalysts for butadiene, isoprene, etc. (see, End. Ing. Chem., 48, 784, 1956; JP 37-008198 B).
On the other hand, in order to attain a higher cis-1,4-linkage content and superior polymerization activity, complex catalyst systems which consist of a rare earth metal compound and an organometallic compound containing a group I to group III element have been studied and developed, and highly stereospecific polymerization has been studied actively (see, Makromol. Chem. Suppl., 4, 61, 1981; J. Polym. Sci., Polym. Chem. Ed., 18, 3345, 1980; DE 2,848,964; Sci. Sinica., 2/3, 734, 1980; Rubber Chem. Technol., 58, 117, 1985, etc.). Among those catalyst systems, complex catalysts containing as main components a neodymium compound and an organoaluminum compound were revealed to provide a high cis-1,4-linkage content and have superior polymerization activity. The catalysts have already been used in industrial applications as polymerization catalysts for butadiene, etc. (see, Macromolecules, 15, 230, 1982; Makromol. Chem., 94, 119, 1981).
With the recent progress of industrial technologies, requirements for polymeric materials as commercial products have become increasingly higher, and development of polymeric materials having still higher thermal characteristics (thermal stability, etc.) and mechanical characteristics (tensile modulus, bending modulus, etc.) has been desired strongly. As one of promising means for achieving the object, attempts have been made to produce a polymer having a high cis-1,4-configuration content in microstructure and a narrow molecular weight distribution by using a catalyst having a high polymerization activity to conjugated dienes.
The inventors of the present invention have found that: conjugated dienes can be efficiently polymerized by using a catalyst composition including a rare earth metal metallocene-type polymerization catalyst and a co-catalyst containing an ionic compound composed of a non-coordinating anion and a cation, and/or an aluminoxane in combination; and a conjugated diene polymer having an extremely high cis-1,4-configuration content in microstructure and a narrow molecular weight distribution can be produced by using the aforementioned catalyst composition for polymerization (see, JP 2000-313710 A).
However, development of a production method for a conjugated diene polymer having a still higher cis-1,4-configuration content in microstructure and a narrow molecular weight distribution has been desired.
Further, an organic solvent such as toluene is known to be toxic to a living body, and a development of a production method for a conjugated diene polymer having the aforementioned characteristics by using an organic solvent without toxicity to the living body is desired.